Britain’s most respected opinion pollster has warned Labour its chances of winning a majority at the next election verge on the “improbable” and that blaming its defeat on a shift away from Blairism is “wholly inadequate”.

John Curtice, the only psephologist to have emerged with an enhanced reputation from the election campaign after his exit poll proved roughly correct, said the election result marked a “fundamental transformation in party competition in the UK”, which made the prospect of another majority Labour government unlikely.

The Strathclyde University politics professor also warned Labour’s leadership contenders that they risk continuing to leave the party outflanked on the left by the Scottish National party by stressing the importance of appealing to aspirational voters.

In a recording of a lecture at Newcastle University, published today, Curtice pointed out that Labour would need a 12.5% lead over the Conservatives just to get a majority at the next election if support for the Scottish nationalists stayed the same.

Spelling out the scale of the electoral mountain the party is facing, Curtice said: “Because Labour has lost so much ground north of the border, what it has to achieve south of the border in order to counteract that is, I would suggest, towards the ends of the improbable.”

If the parties were neck and neck in support, Labour would be almost 50 seats behind the Tories, he added. And just to get the same number of seats would require a Labour lead over the Tories of almost 4%, Curtice showed in a slide to illustrate the lecture.

Before showing the slide Curtice warned “anybody in the room who is a Labour supporter and of nervous disposition” to brace themselves.

Figures from Professor John Curtice showing the lead Labour would need to win a majority in the next election, if the SNP vote remains the same. Photograph: Professor John Curtice

Curtice explained that a shift in the electoral arithmetic has overturned assumptions about Labour’s electoral prospects.

He said the shift involved a Tory tendency to win in marginal seats and a greater distribution of Labour voters in seats where it was pipped by third parties, adding: “It is these last two things which has turned a system that once looked as if it was to the advantage of Labour to the Tories’ advantage, and there is a boundary review to come. It is going to be tough for Labour next time round.”

The distribution of party support in the 2015 election has turned a system that once favoured Labour to the advantage of the Tories, Curtice said. Photograph: Professor John Curtice

Curtice also suggested it would be wrong for Labour’s leadership hopefuls to shift the party to the right. He said: “Events north of the border [show] the Blairite analysis of why Labour lost is frankly wholly inadequate.”

He said the figures on Labour’s losses in Scotland could not be explained by its stance on independence. “It is also about Labour not being seen as the party of equality by voters on the left in Scotland and the SNP capturing that territory,” he said.

Figures from last September showed that of those who had switched from Labour to the SNP, 75% thought the SNP backed policies to redistribute income from the rich to the poor, compared to just 48% who thought Labour backed redistribution.

“The SNP managed to convince Scots that it would create a more equal society,” Curtice said.

He asked: “Where did Labour above all lose in Scotland? Did they lose because they thought the Labour party wasn’t aspirational enough – had gone too far to the left? No. Anybody who is trying to sort out the Labour party now has to take on board that in Scotland ... the problem is the party is not sufficiently far to the left in the eyes of voters.”

Scots who switched support from Labour to the Scottish nationalists felt the SNP favoured redistribution of wealth. Photograph: Professor John Curtice

Curtice, who as president of the British Polling Council launched an inquiry into why opinion polls failed to accurately predict the result, also joked about how his exit poll proved to be correct.

After it was released on 7 May, former Liberal Democrat leader Paddy Ashdown infamously said he would eat his hat if it was correct. Curtice referenced this when he was asked what it felt like to be in an election studio when so many people were sceptical about his figures. He pointed out that he was in a different area of the studio to the sceptics, adding: “Unfortunately it meant that therefore I couldn’t fire any paper aeroplanes at Paddy Ashdown.”